1. Field of the Invention
The present invention relates to a device for cleanly opening the mobile contacts of an on-load current breaking apparatus such for example as a contact maker-breaker comprising at least two fixed contacts and a mobile contact bridge having at least two mobile contacts which are applied against the fixed contacts under the effect of a pole spring in the closed position of the contactor.
2. Description of the Prior Art
It is generally known that, when contact circuits have abnormally high currents flowing therethrough, for example due to a short circuit, the contacts are subjected to repelling forces by electrodynamic effect. This phenomenon occurs in particular in contact circuits comprising fixed contact-carrying conductors bent in the form of a J because of the passage of currents in opposite directions in one of the two legs of the J and in the mobile contact bridge.
Thus, as soon as the current reaches an excess current threshold, these repelling forces overcome the action of the pole spring and cause opening of the contact, which results in breaking the current. With this breaking of the current, the repelling forces disappear so that the mobile contacts pushed by the spring fall back onto the fixed contacts and thus close the circuit again.
In the case of high over currents, since this repulsion is very sharp, the mobile contacts are moved well away from the fixed contacts and the risks of closing again under load are small.
On the other hand, for smaller over currents, the repelling forces occur close to the current peak and, since they are much smaller, when they cease the contacts close again on a current which is still high, so that there is an appreciable risk of welding which is added to the other risks of destruction of the apparatus and the inevitable wear of the contacts.
To overcome these disadvantages, it has been proposed to compensate for the effect of these repelling forces during the time interval separating the application of the over current and opening of the circuit, so as to prevent the fixed and mobile contacts from being separated in an untimely manner, with the consequences which would be caused for certain values of the overcurrent.
This compensation may be achieved, in accordance with the solution proposed in French patent application No. 81 22957 filed in the name of the applicant, by means of a first piece made from a magnetic material having the shape of a U whose base is coupled to the switching assembly of the contactor, and a second magnetic piece integral with the contact carrying mobile bridge, adapted so as to form an air gap with the upper ends of the legs of the U of the first piece, the mobile contact carrying bridge being engaged in the opening of the U of the first piece so as to form a second air gap. With such a device, as long as the intensity of the overload current remains below an overcurrent threshold, the second magnetic piece exerts on the first a force of attraction sufficient to counteract the effect of the repellent forces. Beyond this over current threshold, the force exerted by the compensator reaches a limit value due to the saturation of the magnetic material pieces, whereas the repellent forces between the contacts continue to increase, so that opening of the contact occurs. However, in such a device, there exist operating zones in which the resultant force on the contact bridge varies slowly as a function of the overcurrent values of the current flowing in the current bridge. If this small variation occurs towards cancellation of the contacting force, the opening speed thereof will be low, which will cause risks of cut-off failure of the type described above.
A device is further known, more especially from French patent application No. 81 15606 filed in the name of the applicant, for releasing the mobile contacts of the contactors, so as to limit the shore circuit currents. This device uses more especially a threshold coupling between the mobile contact bridge and the switching assembly which is associated therewith. In this device, the threshold coupling is formed by balls housed directly in a rod integral with the mobile contact bridge and cooperating with a resilient clamp connected to the switching assembly. It is then apparent that, in this device, the pole force causes a permanent stress on the threshold coupling in the working condition even when the current is less than the normal current of use. This stress, combined with the high number of switching operations which the apparatus must make under normal conditions of use, causes fatigue of the coupling which may lead to a considerable drift of the longitudinal holding force and thus considerable dispersion of the overcurrent values causing disengagement of the coupling.
The purpose of the invention is then to overcome all these disadvantages. It provides a contact using a device for compensating the repellent forces of a type similar to the one described in the above French patent application No. 81 22957 but in which, when the electrodynamic repellent force due to an abnormal over current exceeds a predetermined value, opening of the contacts is obtained with a considerably increased speed and is maintained at least temporarily.